In general, an airborne ultrasonic sensor is a device that serves to transmit ultrasonic waves into the air and receive the ultrasonic waves which have been reflected by an object, wherein it is applied to a variety of kinds of fields such as one for vehicle mounting, etc.
First, as a conventional device, there has been proposed an obstacle detection device for mounting on a vehicle, which is provided with an airborne ultrasonic sensor (for example, see a first patent document).
In the conventional device described in the first patent document, in place of a structure in which a bumper of a vehicle is slotted to expose an acoustic radiation surface, a concave portion is formed in a rear surface of the bumper, and a sensor is arranged in the concave portion, thereby preventing the appearance of the bumper from being spoiled due to the formation of the sensor in the concave portion.
However, according to the above-mentioned first patent document, the bumper and the sensor are unified with each other, so there is a possibility that the sensor may receive spurious waves propagating through the bumper.
Accordingly, in order to prevent the reception of such spurious waves, there has also been proposed a device which has a groove portion formed in a bumper (for example, see a second patent document).
In the obstacle detection device described in the second patent document, the construction thereof is such that the groove portion and a protruded portion are formed in the surroundings of a sensor which is arranged on the rear surface of the bumper, thereby preventing spurious signals due to vibrations from being transmitted or received.
However, according to the above-mentioned second patent document, no assumption is made on the spurious waves which would be generated by reflection waves from a reflection source being incident to the bumper, and hence, in actuality, there can also be well considered a situation where the spurious waves generated by the incidence of the reflection waves to the bumper propagate through the bumper, so that they may be received by the sensor.
Here, reference will be made to a propagation path of spurious waves and influences due to the spurious waves in an airborne ultrasonic sensor, while referring to FIG. 7.
FIG. 7 is a plan view showing the airborne ultrasonic sensor in the past.
In FIG. 7, a sensor body 1 is mounted on a rear surface of a housing (a bumper in the case of the airborne ultrasonic sensor for vehicle mounting) 20.
The transmission and reception device 10 for electrical signals is connected to the sensor body 1, and the sensor body 1 is caused to excite by an excitation signal which is inputted from the transmission and reception device 10, thus sending ultrasonic waves.
The ultrasonic waves sent by the sensor body 1 propagate through the housing 20 from a radiation surface 20a thereof to the outside.
The ultrasonic waves having propagated to the outside through the housing 20 are reflected by a reflection source 3 (a surrounding object), so that a part (spurious wave) of the reflected waves is received by the sensor body 1 through a path R1 and the housing 20, and is converted into an electrical signal.
On the other hand, the reflected waves from the reflection source 3 propagate not only in the direction of the sensor body 1, but also in a wide range of directions, and hence, they of course propagate in directions different from the direction of the sensor body 1, too, and there also exist reflected waves (spurious waves) which vibrate the housing 20, as shown in a path R2.
When the housing 20 vibrates, the vibration (see a wave arrow of a broken line) will propagate through the housing 20, and will arrive at the sensor body 1.
That is, as propagation paths through which the reflected waves from the reflection source 3 are received by the sensor body 1, there exist two paths including the path R1 (in which the reflected waves propagate directly in the direction of the sensor body 1 so as to be received thereby), and the path R2 (in which the vibration of the housing 20 propagates through the housing 20 so as to be received by the sensor body 1).
In cases where the two propagation paths exist as mentioned above, when a difference between the propagation paths is “a natural number times of the wave length”, the reception signals received through the two propagation paths act to strengthen each other, but on the contrary, when the difference is “a natural number times of the wave length+a half wave length”, the reception signals received through the two propagation paths will mutually weaken each other.
That is, when consideration is given to the interference state of the two reception signals including the spurious waves, in cases where the difference between the propagation paths is one wave length, the two reception signals interfere with each other to strengthen each other, so that the amplitude of the composite or synthesized wave thereof becomes larger. On the other hand, in cases where the difference between the propagation paths is a half wave length, the two reception signals interfere with each other to weaken each other, so that the amplitude of the composite wave becomes smaller.
For example, under the conditions that the difference between the propagation paths is one wave length, and that the amplitudes of the two reception signals are made to strengthen each other, in cases where the airborne ultrasonic sensor is applied to a system which estimates the size of the reflection source 3 according to the amplitude of the composite wave, the size of the reflection source 4 may be overestimated.
On the other hand, when the difference between the propagation paths is a half wave length and the amplitude of the composite wave becomes smaller, it may, as a result, become difficult to detect the reflection source 3.
In addition, even in cases where the two reception signals do not interfere with each other, the vibration duration time of the composite wave becomes longer, as a result of which the resolution of the airborne ultrasonic sensor may be degraded.